1. Field of the Invention
This invention relates to a frequency doubler for a microwave band which uses field effect transistors device.
2. Discussion of Background
A frequency doubler for a microwave band is used to double a high-quality oscillation output for a low frequency when a high-frequency signal which satisfies a required output level, frequency stability, noise characteristic, etc. cannot be obtained in a direct oscillation mode.
The frequency doubler becomes more and more important due to the use of a high frequency wave of the order of, for example, millimeters in a communication field and so on.
A frequency doubler using a field effect transistor (hereinafter referred to as an FET) device is higher in conversion gain than a frequency doubler using, for example, a step recovery diode or a varactor diode, and suitable for a monolithic microwave integrated circuit (hereinafter referred to as an MMIC) which is widely expected to be a rapidly growing field. For this reason, MMICs can be used as a dominant doubler for a microwave band and thus will have many practical applications. Since such a doubler allows the use of either single gate FETs or dual gate FET structure and operates on substantially the same principle, a conventional doubler using the dual gate FET structure will be explained below with reference to FIG. 1.
FIG. 1 shows an equivalent circuit of a dual gate FET structure comprised of a cascade connection of two single gate FETs 1 and 2, in which the gate of first FET 1 corresponds to a first gate of the dual gate FET structure and the gate of second FET 2 corresponds to a second gate of the dual gate FET structure. In order to perform a frequency-doubling function, an input signal of a fundamental frequency fo is input to the first gate of the dual gate FET structure so that the drain current waveform is deformed due to the nonlinear effect of the dual gate FET structure. Of the deformed current wave, only a double-frequency component is selected through band-pass filter 3 connected to the drain of the dual gate FET structure and is delivered as a double frequency 2fo through matching circuit 4. The gate of second FET 2 is grounded through impedance 5. Gate biases Vg1 and Vg2 are applied respectively through frequency choking coils 6 and 7 to the gates of first and second FETs 1 and 2. A drain bias Vd is supplied through frequency choking coil 8 to the drain of second FET 2.
If the current distortion mechanism of the aforementioned circuit arrangement is explained in connection with the single gate FETs, then three kinds of modes are involved as shown, for example, in FIGS. 2, 3 and 4.
That is, in the mode as shown in FIG. 2 a drain current Id is deformed by greatly varying a drain-to-source voltage Vds with a gate-to-source voltage Vgs fixed; in the mode as shown in FIG. 3 a drain current deformation is produced due to a pinch-off effect produced when the gate-to-source voltage Vgs varies in the neighborhood of a pinch-off voltage Vp; and in the mode as shown in FIG. 4 the gate-to-source voltage Vgs varies in the neighborhood of a built-in voltage V.phi., saturating a drain current.
The operation of the doubler using the dual gate FET structure becomes very complex due to a mutual operational relation of two single gates FET and to the involvement of the three kinds of modes. In order to attain a maximal conversion gain it is necessary to adjust the fundamental power level of the bias voltages of the first and second gates of the dual gate FET structure, etc. to their optimal levels.
As we think evident from the above, the conventional doubler operates on the principle that deformation is induced in the drain current of the dual gate FET structure and a double-frequency component is selectively taken therefrom. Considered from the standpoint of the conversion gain, the frequency doubling of the fundamental wave is not necessarily implemented with maximum efficiency; there remains room for improvement. The drain current contains many unwanted frequency components, such as the fundamental wave, triple frequency wave, quadruple frequency wave, . . . in addition to the desirable double frequency wave. In order to eliminate these unwanted frequency waves it is necessary to employ a band-pass filter with an excellent selection function. Thus a bulkier circuit results. The MMIC whose manufacturing cost is proportional to the area of a chip is not desirable from the standpoint of economy.